The medical field commonly employs a wide range of devices that depend upon the transmission of signals to monitor or measure various biological or environmental parameters of a patient. The signals may be transmitted and measurements made of the signal as reflected from or transmitted through, for example, tissue or an organ, or the signal may be used to communicate measured data rather than to obtain the data.
For example, a common example of devices that employ the transmission and reception of signals in the measurement of one or more biological or environmental parameters of a patient are the various forms of blood oximetry devices. As is well known, blood oximetry devices are commonly used to monitor or measure the oxygen saturation levels of blood in a body organ or tissues, including blood vessels, or the oxidative metabolism of tissues or organ. Such devices are often capable of and are used to determine pulse rate and volume of blood flow in organs or tissues, or to monitor or measure other biological or environmental parameters.
Blood oximetry devices may be considered as exemplary medical devices using the transmission or reflection of signals to gain or communication information, and as fairly illustrative of the problems of the prior art.
As is well known to those of skill in the arts, blood oximetry devices measure the levels of the components of one or more signals of one of more frequencies as transmitted through or reflected from tissue or an organ to determine one or more biological or environmental parameters, such as blood oxygenation level and blood volume or pulse rate of a patient. Blood oximetry devices may also be constructed as directly connected devices, that is, devices that are directly connected to a patient and that directly present the desired information or directly record the information. It is also well understood, however, that blood oximetry devices may also be implemented as remote devices, that is, devices attached to or implanted in a patient and transmitting the measurements to a remote display, monitoring or data collection device.
Considering blood oximetry devices in further detail as representative and exemplary of a wide range of medical devices, oximetry devices measure blood oxygen levels, pulse rate and volume of blood flow by emitting radiation in a frequency range, such as the red or near infrared range, wherein the transmission of the radiation through or reflectance of the radiation from the tissues or organ is measurably affected by the oxygen saturation levels and volume of the blood in the tissues or organ. A measurement of the signal level transmitted through a tissue or organ or reflected from a tissue or organ may then provide a measurement or indication of the oxygen saturation level in the tissue or organ.
The transmitted or reflected signals may be of different frequencies which are typically affected in measurably different ways or amounts by various parameters or factors or components of the blood. The conjunctive use of signals at different frequencies, which may be close in frequency, or wavelength, may in turn provide concurrent representations of multiple factors or parameters which may be very different from one another which may be closely related, such as the level of oxygen in the blood in conjunction and a reference for the blood oxygen level measurement. It should also be noted that the parameters or factors represented by transmitted or reflected signals may be represented by different and related or unrelated parameters of the received signals. For example, a signal transmitted through or reflected from tissue or an organ to measure, for example, blood oxygenation or flow, may have a constant or “dc” component due to the steady state volume of blood in the tissue or organ and a time varying or “ac” component indicative of the time varying volume of blood flowing through the tissue or organ due to the heart beat of the body. Each signal component may provide different information, and may provide information that may be used together to generate or determine yet other information.
An example of an oximetry device is described in U.S. Pat. No. 4,281,645 to Jobsis for METHOD AND APPARATUS FOR MONITORING METABOLISM IN BODY ORGANS, which describes an oximetry system which continuously measures the oxidative metabolism of an organ by transmitting alternating reference and measurement light signals through the organ. The device adjusts the power level, or gain, of the receiving photomultiplier to maintain the output signal level generated by the photomultiplier during the reference signal transmission at a predetermined level, and then maintains this amplifier gain level during transmission of the measurement signal. The result is that the photomultiplier gain is automatically compensated for changes in blood volume in the organ, and the amplifier gain control signal reflects and is used as a measurement of blood volume in the organ.
In yet another example, U.S. Pat. No. 4,653,498 to New, Jr. et al. for PULSE OXIMETER MONITOR, describes a pulse oximeter system wherein the power levels of the diodes emitting a reference signal and a measurement signal are adjusted, by measurement of the received reference signal, to provide received signals within the acceptable input voltage range of a digital to analog converter that converts the received signals into measurement indications.
In a further example, U.S. Pat. No. 4,859,057 to Taylor et al. for OXIMETER APPARATUS describes a reflectance oximetry apparatus which transmits a red signal and an infrared signal and determines the dc and pulsating components of the reflected return signals wherein the pulsating component represents the level of blood oxygen. The emitted power of the red and infrared LEDs are controlled to provide a relatively stable level of dc component.
U.S. Pat. No. 5,069,214 to Samaras et al. for FLASH REFLECTANCE OXIMETER describes a flash reflectance oximeter that employs short duration, high intensity red and infrared measurement and reference pulses to allow oximetric measurements through barriers such as clothing or protective wraps. The duration and intensity of the pulses are adjustable by the user to accommodate different thicknesses of material.
In final examples, U.S. Pat. Nos. 5,924,979 and 5,746,697 to Swedlow et al. for MEDICAL DIAGNOSTIC APPARATUS WITH SLEEP MODE describe a medical diagnostic apparatus wherein the apparatus enters a “sleep mode” to conserve power when monitored physiological parameters have been stable and within a selected range for a predetermined period, and are “awakened” for further measurements after selected periods.
The use of yet other devices or systems that rely upon the transmission and reception of some form of signal or signals to detect, monitor or measure yet other biological or environmental parameters of a patient are also well known to those of skill in the arts. For example, many other types of biological or environmental parameter monitoring or measuring device or systems use signals transmitted or reflected through or from tissue or an organ to detect and measure various biological or environmental parameters or may transmit biological or environmental data from a data collection or monitoring device, such as an implanted or remote cardiac monitor, to a data collection device or system. Still other devices or systems, such as an implanted or remote blood oximetry device, may employ transmitted or reflected signals to measure a biological or environmental parameter, such as blood oxygen level, and yet other signals to communicate the information to a remote data receiving device or site. The adaptation of and advantages of the present invention to and in such devices will, however, be well understood by those of skill in the arts.
More recent developments in medical devices, and in particular in medical monitoring and parameter measuring and data collection or monitoring devices, such as blood oximeters, has been in the direction of smaller, lighter and more portable devices. Such “miniaturized” devices may be used, for example, for remote or portable use, such as by emergency response teams, or as individual user devices rather than as devices shared among several users, and are generally more convenient even in a hospital setting as requiring less room for storage and less space when in use. An accompanying trend has been for the combination of two or more devices of different types, which are typically related in some way in function or in use, into a single devices. Yet another trend in recently developed devices is the provision of increased data or signal processing power and data storage capacity, such as required for advanced algorithmic processing.
The development of such smaller and more portable devices, however, has meant greater reliance on smaller, more portable or more convenient power sources to drive the devices, such as batteries as opposed to connections to power lines. This trend, in turn, has led to greater concerns regarding power consumption and battery life of the devices. For example, in a typical blood oximetry device or system, up to 50% or more of the power consumption of the device is used in driving the light sources, which are typically light emitting diodes (LEDs) generating the red or infrared signals that are transmitted through or reflected from the tissue or organ to measure the levels of oxygen in the tissue or organ.
It will be apparent from the above examples of blood oximetry devices, however, that power consumption has not, until recently, been a concern in the design of most medical devices, including blood oximetry devices. For example, It will be noted from the above examples of blood oximetry devices that the oximetry devices of the prior art are, in general, designed to control either the signal level of the transmitted light or the amplification or gain of the receiving circuits so as to provide a received signal of sufficient amplitude and signal to noise ratio to support an analysis providing an acceptably high confidence level. As described in the above cited example, the oximetry devices of the prior art are thereby directed towards increasing the received signal level by either increasing the transmitted signal level or increasing the gain or amplification of the received signal, both of which increase power consumption of the devices. Yet other devices and systems, including those relying upon the transmission and reception of signals for data communication are likewise designed and optimized to increase the received signal level and the received signal “signal to noise” ratio.
There has been some development, however, in providing more efficient LEDs, such as the OxiMax sensors available from Nellcor, that provide the higher levels of brightness for equal or lower power consumption, or in approaches such as that discussed in Swedlow '979 and '697, wherein the device is “put to sleep” for periods predicted on the period or rate of possible change of the measured parameter. LEDs, however, are not suitable or usable for all devices, and may not be able to generate light signals are the desired wavelengths. Also, the method of “putting a device to sleep” based upon a predicted “safe period” before change of a parameter will not be acceptable in all instances. In many instances, continuous monitoring will be necessary or desirable due to the significance of the parameter being measured or monitored or the risks represented or reflected in the parameter or, in some instances, the period or rate of change of the parameter may be unpredictable or too short to allow a “sleep” period. Yet other approaches of the prior art for power reduction include reducing the level of the emitted signal, as is suggested for other reasons in Samaras '214. It must be noted, however, that such power reductions are, as in Samaras '214, typically for other reasons, such as avoiding burning the patient, and that such methods are risky or unacceptable because a reduction of emitted signal power may result in the desired information in the signal becoming buried in environmental and system noise, or being degraded to the point of being useless or even hazardous.